Once on the site, you pick your model and carrier. Then you select how much data space your phone holds and what condition it is in.

Then you get an estimate.

Here is an estimated range of what you can expect. If you have an iPhone 4S, with AT&T and 64 gigabytes of space, you could get about $315 to trade in your phone. If you have an iPhone 4, with less storage space, you're in the $140 range. If you're on the low end, like an iPhone 3GS with only 8 gigabytes, you might only get about $30.